1. Field of the Invention
This invention relates to long impression life, laser imageable lithographic printing plates and to the method for their production. More particularly, this invention relates to lithographic printing plates for wet and waterless offset lithographic printing which can be imagewise exposed using a digitally controlled infrared laser.
2. Description of Related Art
Lithography and offset printing methods have long been combined in a compatible marriage of great convenience for the printing industry for economical high speed, high quality image duplication in small runs and large. Known art available to the industry for image transfer to a lithographic plate is voluminous but dominated by the photochemical process wherein a hydrophilic plate is coated with a photosensitive coating, exposed via a film image and developed to produce a printable, oleophilic image on the plate for use in traditional wet lithographic printing processes employing an aqueous fountain solution. Alternatively, waterless lithographic printing plates, i.e., plates that require no fountain solution, have been developed wherein a plate is photochemically produced which has oleophilic image areas and complimentary areas which are both hydrophobic and oleophobic. Such waterless plates overcome difficulties typically encountered with the traditional wet process such as the unwanted mixing and emulsification of fountain solution and ink.
With the advent of electronically controlled laser exposure systems, there is an industry trend to directly image lithographic plates by such systems instead of using the time consuming process of producing traditional litho films for imaging the plates. With such laser exposure systems, a plate is exposed by a digitally modulated laser which is scanned across the surface of the sensitive plate. However, traditional lithographic printing plates are imaged by ultraviolet radiation, whereas most lasers have output radiation in the visible and infrared spectral region. To overcome this spectral mismatch, lithographic plate structures have been developed which are sensitive to conventional laser radiation. One class of laser sensitive plates provides laser sensitivity to an oleophilic layer over the hydrophilic plate such as disclosed in U.S. Pat. No. 5,340,699, European Patent Publication 599510, and International Publication WO 20429/96. Such single layer plates which typically are sensitive to infrared lasers represent a compromise between printing performance and laser sensitivity and require additional heating or curing steps to provide an acceptable printing image. Another class of laser sensitive plates are composed of a conventional photosensitive lithographic plate which has a laser sensitive mask forming layer over the photosensitive layer of the plate such as those disclosed in U.S. Pat. Nos. 5,330,875 and 5,512,420 wherein the mask layer is a silver halide emulsion, and International Publication WO 97/00777 wherein the mask layer is a thermal ablation mask. While such laser sensitive mask/plate systems produce plates with conventional printing performance, such silver halide systems are expensive to make and process and must be handled in a dark room under dim red light; and such ablative masks require very high laser exposure doses resulting in slow imaging speed.
There continues to be a need for high speed, low cost laser imagable lithographic printing plates which are stable prior to exposure and have at least conventional printing performance.